This invention relates generally to medical imaging systems, and more specifically to a method and apparatus for quantifying tissue fat content using a medical imaging system.
In spite of recent advancements in computed tomography (CT) technology, such as faster scanning speed, larger coverage with multiple detector rows, and thinner slices, energy resolution is still a missing piece. Namely, a wide x-ray photon energy spectrum from the x-ray source and a lack of energy resolution from CT detection systems preclude energy discrimination CT.
X-ray attenuation through a given object is not a constant. Rather, x-ray attenuation is strongly dependent on the x-ray photon energy. This physical phenomenon manifests itself in an image as a beam-hardening artifact, such as non-uniformity, shading, and streaks. Some beam-hardening artifacts can be easily corrected, but others may be more difficult to correct. In general, known methods to correct beam hardening artifacts include water calibration, which includes calibrating each CT machine to remove beam hardening from materials similar to water, and iterative bone correction, wherein bones are separated in the first-pass image then correcting for beam hardening from bones in the second-pass. However, beam hardening from materials other than water and bone, such as metals and contrast agents, may be difficult to correct. In addition, even with the above described correction methods, conventional CT does not provide quantitative image values. Rather, the same material at different locations often shows different CT numbers.
Another drawback of conventional CT is a lack of material characterization. For example, a highly attenuating material with a low density can result in the same CT number in the image as a less attenuating material with a high density. Thus, there is little or no information about the material composition of a scanned object based solely on the CT number. Additionally, detection of fat within tissues is often difficult because the images produced by such scanners may exhibit a significant level of image artifacts and CT number inaccuracy. These limitations may prevent the utilization of the CT device for advanced diagnosis. For example, some normal and pathological biological processes result in accumulation of higher levels of fat within an organ or tissue. In some cases, the fat can be readily detected by imaging or physical examination. In other cases, the fat is distributed throughout normal tissue in varying amounts, possibly making detection of the distributed fat difficult.
An example of fat distributed throughout normal tissue is the accumulation of fat in liver cells, herein referred to as a fatty liver. A fatty liver is typically not considered a disease because it does not damage the liver, however, a fatty liver is symptomatic of a number of pathological processes including, for example, tuberculosis, diabetes mellitus, extreme weight gain, alcoholism, poor diet, intestinal bypass surgery for obesity, and the use of certain drugs such as corticosteroids. Known methods for diagnosing a fatty liver include microscopically examining a sample of liver tissues obtained from biopsy of the liver. In addition, both a bright ripple pattern as seen on an ultrasound image of a liver, and reduced density as seen on an x-ray computed tomography (CT) image of a liver, may suggest the presence of a fatty liver. Furthermore, fatty liver patients may have an enlarged liver or have isolated minor elevation of liver enzymes as measured by routine blood screening. However, a definitive diagnosis of a fatty liver is typically determined by microscopically examining liver tissue samples obtained from an invasive liver biopsy procedure.